For a simple bomb you want Uranium, Plutonium emits too many neutrons which are likely to cause a small chain reaction while your critical mass is in the process of forming and this predetonation can then blow everything apart before a proper critical mass has been formed. A gun type design where you just smash two subcritical lumps of metal into each other is, as I remember, nearly impossible to achieve with Plutonium you can realistically produce.

EDIT: Interestingly another commenter says that this actually was a Plutonium design.